aircraft window material selection 3

[tstanley]

Are many aircraft windows made of stretched acrylic? I've read a thread here that described the 747 window as a laminate with a thin layer of glass on the outsides. My question regards the choice between stretched acrylic and polycarbonate. The stretched acrylic is stronger and stiffer than polycarbonate, but is much more notch sensitive as shown by the Izod impact test. How do the manufacturers using streched acrylic overcome this problem, both in manufacturing and later maintenance. Is this why they use the laminated windows?

Tom Stanley

 

[rb1957]

a window manufacturer (sierracin) will give you the best answer. one thing to consieder is are you looking at a passenger window or the flight compartment windscreen (which'll have erosion issues as well as bird strike). i've seen both types of window (acrylic and polycarbonate) usually multiple plies. i would suspect that surface erosion is one reason for multile plies.

 

[tstanley]

Thank you for the help. I checked their web site and saw that for cessna aircraft they use the laminated acrylic and put in sleeves for the bolts so as to minimise the chance of a notch (burr) at the hole.

The question actually arises from powerboat racing. They look to the aircraft industry for their technology but they don't have the rigorus standards and quality control before and after manufacture that the aircraft industry does. There has been a long term controversy about the best material for their windshields and the bird strike test is often referred to, although their incidents are somewhat different.

I was wondering if the aircraft industry also had the controversy but it looks as though they can minimize the probability of developing small stress raisers leading to the notch sensitivity problem.

 

[rb1957]

i'd've thought that power boat racing would have looked for the least weight solution ... that could be a helmet viser (but spray is an obvious problem !).

bird-strike ? maybe a heavier bird, but impact velocity is much less.

stress raisers ? ... fatigue ?

 

[thruthefence]

Consider the pressurization loads on those Cessna windows, oversized holes with bushings. I don't think your boat will see those.

 

[tstanley]

The problem comes when the boat lands upside down in the water at over 100 mph. We are trying to protect the driver in this situation. The windshield plays a key role in this. Many are similar to what is shown by the sierracin web site as for a cessna, at the current time.

The key question on the material choice is whether to go for the better stiffness and strenth of the stretched acrylic, or a better notch sensitivity with the polycarbonate. I'm assuming with aircraft there are strict standards regarding hole preperation for bolting of the windshield into the aircraft frame. Correct?

Thanks for your help.

Tom Stanley

 

[TVP]

Tom,

It's not just notch sensitivity-- it is the entire spectrum of fracture toughness, meaning crack initiation, crack propagation, etc. Polycarbonate is substantially tougher than acrylic, and will outperform acrylic in the type of dynamic (i.e. high strain rate) event you just described.

 

[rb1957]

intuitively i'd start with windscreen build-ups; but manufacturers will be better intuition (and data !). Appreciating your goal (crashworthiness and survivability) would have helped making usefull suggestions. my 2c ... probably setting the windscreen as a deflector (to deflect the water) rather than as a barrier (to push the water away from the cockpit) will probably go more towards survivablity than the strength/stiffness of the windscreen panel. which suggests modelling and tests to validate.

 

[WKTaylor]

tstanley... be cautious!.

The windshield pane(s) is(are) only 1/2 the picture: the windshield frame and substructure is the other 1/2 of the picture. The frame/substructure is essential for transmitting shock windshield loading to the forward hull... without the pane shooting out of the frame. Crushing loads due to inverted contact with water will be a bear to handle!

Comments.

Acrylic-polycarbonate-polycarbonate-acrylic laminates are a good match for what You are doing.

The acrylic inner and outer plys provide high quality abrasion/erosion protection for the structural (soft/tough)polycarbonate structural plies. In addition, acrylic [or an acrylic replacement], has the ability to accept long-lasting abrasion/scratch resistant coatings. DO not accept a polycarbonate windshield with JUST hard coatings: Repair can be impossible. Also, do not accept a wndshield with only one acrylic ply: the opposite side of the pane is will be abrasion prone (repair is difficult-to-impossble to accomplish).

NOTE.
May want to conside an antiglare coating and a hydrophobic coating if sunlight-glare and water spray are significant factors.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[tstanley]

Thanks for your very informative posts. The comment about the fracture toughness and crack propagation is particularly interesting. I'm hearing that regardless of the material, the windshield should be laminated and that the combination of materials as Wil Taylor suggests is probably the best. It seems to me that the original F-16 canopy had a similar combination.

Regarding the frame, I have done a couple of fea models of the entire canopy, including the windshield. Using that tool the frame around the windshield can be strengthened. Right now though, the fea shows the windshield itself is the weak point.

In the slower classes, of course, cost becomes a factor. When cost will only allow a single ply we've been recommending the polycarbonate with the hard coatings. Some builders still insist on using a single layer of stretched acrylic though. That's when I start to worry about the notch sensitivity and now fracture toughness.

Tom Stanley

 

[rb1957]

could it come down to class rules ? no-one interested in the competition wants to see a driver hurt. F1 have made major advances in safety (albeit after some sifnigicant accidents). From my experience, modelling a dynamic impact is notoriously (infamously) difficult to get right, which is why we still rely on bird guns. your impact is especially difficult (i think) 'cause you have a less not-very-well-contained impacting mass ... i'm thinking that the water will spread more than the bird's inards ?

 

[WKTaylor]

OH Yeah rb1957 reminded me...

The greater the slant angle of the windshield the better... up to the point it becomes too wide and hard to manufacture and structurally support .. or becomes too heavy.

Also ensure that when roll-over occurs, the windshield can't inadvertently turn sideways and become a water-scoop. A semi-enclosed cockpit with side shields may be required.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[tstanley]

rb1957 you are correct in that this is aimed if not at the actual rules but in disseminating the information to improve the knowledge prior to updating the rules. No one wants to see people get hurt.

Mr. Taylor is also right in that slanting the windshield will help up to a point. The equations are well known for when the boat is running right side up. So some approximations can be done for the reverse. I don't have the software to find out more accurate forces through simulation. Once again it is a compromise between reducing the forces by slanting the windshield, keeping the span small to reduce stress, and giving the driver enough vision. The cockpits are fully enclosed as much as possible, with hatches for entry and egress (a different problem)

It is sure great to discuss this with knowledgable people willing to help. I appreciate it.

Tom Stanley

 

[rb1957]

to a point, why have a windscreen ? why not video cameras ? and use the entrance hatch as an emergency backup.

 

[tstanley]

That is an interesting idea. I don't think it would be accepted for the front windows,though, part of racing is the fun of seeing yourself pass the competitors. However, it might be interesting to offer it in place of the side windows.

Tom Stanley

 

[CasaCEO]

Next window surface will be clear sapphire at least by military and aerospace organizations. Surprisingly they can make rods and wafers in just a couple days that can be formed into larger windows. (Sapphire is the second hardest gemstone and is naturally clear, blue is from metals in the formation of the crystal such as iron.)

 

[KirbyWan]

Transparent Aluminum.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

"sapphire windows" ... that just sounds expensive !

 

[MiketheEngineer]

Don't forget about those new "jumping" fish. A number of people have been seriously hurt by them!!